Adding pieces to the puzzling plant nuclear envelope

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The nuclear envelope (NE) and the nuclear pores are important structures that both separate and selectively connect the nucleoplasm and the cytoplasm. NE and nuclear pore research in plants have recently seen an elevated level of interest. This is based both on new findings demonstrating the importance of nucleocytoplasmic trafficking for several signal transduction events, and on increasing evidence that NE and nuclear pore components play important roles during plant cell division. Here, we review the most recent reports in the field and compare them to the more advanced knowledge about yeast and animal model systems. They deal with the refined ultrastructure of the NE and NPC, with the discovery of novel NE components, and, importantly, with novel roles and fates of NE-associated and NPC-associated proteins during plant mitosis and cytokinesis.

Introduction

In eukaryotic cells, the nuclear envelope (NE) separates the nucleoplasm from the cytoplasm. The gateways for macromolecules through the NE are the nuclear pore complexes (NPCs), large multiprotein complexes of an eightfold symmetry embedded in the NE double membranes. Proteins and RNAs traffic through the NPCs, enabled by the interacting activities of the nuclear transport receptors (the karyopherins), the NPC proteins (Nups) and the elements of the Ran cycle (see Box 1, Figure I). In the animal and yeast fields, a combination of proteome, interactome, functional, and modeling studies are increasingly refining our understanding of the NPC (reviewed in [1]). Plant NE and NPC research is by comparison less advanced, in part based on the old expectation in the field that not much new could be learned by studying such a likely conserved cellular system. Over the past years, however, numerous reports have demonstrated that firstly, plants do not simply possess a carbon copy of the well-known animal and yeast systems [2, 3, 4, 5], and secondly, that both in plants [6] and in animals [7], NE and NPC biology hold more clues about regulation at the cellular and organismal level than previously anticipated. This renewed interest within the plant community is reflected in the number of exciting reports that have appeared in the past 18 months. They deal with the refined ultrastructure of the NE and NPC, with the discovery of novel NE components, and, importantly, with novel roles and fates of NE-associated and NPC-associated proteins during plant mitosis and cytokinesis (Table 1). Here, we focus on these new studies, while referring to recent reviews for plant work before the discussed time period [6, 8].

Section snippets

Ion channels

In animal cells, Ca2+ channels in the NE and ER are known to cause Ca2+ oscillations at the nuclear periphery. The channels are activated by secondary messengers such as IP3, which is liberated by phospholipase C [9]. In plants, Nod factor production by rhizobia causes two genetically separable Ca2+ responses, a rapid influx of calcium into root hair tips, and a slower appearance of calcium oscillations in the perinuclear region [10]. CASTOR and POLLUX were identified in mutant screens in Lotus

Conclusions

Much progress has been made in the past two to three years in determining components of the plant NPC and NE and in describing their contribution to diverse plant signaling processes (reviewed in [6]). In the meantime, however, the field has all but exploded in yeast and animal models, including precise structural models of the NPC, biophysical understanding of nucleocytoplasmic transport, the role of the NE in gene regulation and human disease, and the emerging evidence that nucleocytoplasmic

References and recommended reading

Papers of particular interest published within the period of review have been highlighted as:

  • • of special interest

  • •• of outstanding interest

Acknowledgement

Funding of our lab's research in this field by the National Science Foundation is greatly acknowledged.

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